1. Field of the Invention
This invention relates to apparatus for controlling an hydraulic pump system.
2. Description of the Related Art
Electrically powered hydraulic pump systems are commonly used on tailgate loaders and small dump truck hoists. One reason for this is that a small transmission may not support a power take-off opening. Another reason is that the operation of the hydraulic pump may be required when the prime mover is not running. These electrical pump systems use the twelve volt vehicle electrical battery for power and for control.
Known systems rely on a low current solenoid to engage a high current switch between the battery and an electric motor which drives the pump. A second such solenoid is used to open an hydraulic lowering valve. The solenoids have a spring return for opening the switch between battery and motor and closing the hydraulic lowering valve.
The solenoids, valve, and switch operate in a very hostile environment containing significant amounts of water and salt. This increases the maintenance requirements for the solenoids and also suggests a solenoid design which maximizes the actuating force provided to better ensure reliable operation. However, a higher power solenoid results in large current drains and high heat in the solenoid windings. And even so, the forces available from twelve volt solenoids are relatively low. To lower the force requirements for the hydraulic lowering valve, it is known to use a pilot valve or a small orifice valve. Unfortunately, this solution results in valves that are extremely sensitive to dirt and moisture, especially in cold temperatures when moisture droplets can freeze.
This invention seeks to provide a reliable and robust hydraulic pump system.